1. Field of the Invention
The present invention relates generally to a manufacturing process for producing shadow masks for use in color cathode ray tubes, etc., and more specifically, to a manufacturing process for producing shadow masks by joining a plurality of shadow mask plates stacked upon each other, and a shadow mask used in the manufacturing process.
2. Description of the Related Art
A color cathode ray tube in FIG. 1 includes an electron gun 1 for producing three electron beams B, a fluorescent material 2 arranged on a face plate for receiving the electron beams B produced by the electron gun 1 to give off three primary colors, and a shadow mask 3 arranged between the fluorescent material 2 and the electron gun 1, having a plurality of apertures formed therein for passing selectively only an electron beam in a desired direction among the electron beams B and shielding electron beams in undesired directions.
The shadow mask 3 for a color cathode ray tube is generally manufactured by a process shown in FIG. 2. Referring to FIG. 2, in the first step, a piece of low carbon aluminum killed steel or an invar alloy having a thickness between about 0.1 and 0.3 mm is prepared for a shadow mask plate. The invar alloy is, for example, an iron-nickel alloy containing 36% nickel by weight.
In the second step, a plurality of apertures are formed in the shadow mask plate by a photoetching process.
After the etching process, annealing is performed on the shadow mask plate having the plurality of apertures, for the purpose of providing the shadow mask plate with press molding applicability. The annealing is performed as follows: The shadow mask plates are lifted or piled upon each other in an oxygen-free atmosphere. The shadow mask plates formed of aluminum killed steel are heated at a temperature between about 700.degree. C. and 900.degree. C., while the shadow mask plate formed of an invar alloy are heated at a temperature around 1000.degree. C. The heating of the shadow mask plate allows its yielding point, i.e. strength to be decreased, and the shadow mask plate is provided with press molding applicability. The temperature of annealing varies with the kind of material used for the shadow mask plate. In the case of the invar alloy, if the temperature of annealing is below a predetermined value, the shadow mask plate remains partially elastic. In that case, strength to return to its original shape remains in the shadow mask plate, hampering its press molding.
The annealed shadow mask is pressed into a prescribed curvature, for example into a sphere. The shadow mask is blackened in a blackening furnace for the purpose of improving its property of heat radiation and reducing the irregular refraction of electron beams, and an oxide layer is formed on the surface thereof. This process completes the manufacturing of the shadow mask.
As is well known, in a color cathode ray tube, a large number of electron beams produced by the electron gun impinge upon the shadow mask and are absorbed thereto. The energy of these absorbed beams is converted into thermal energy on the shadow mask whereby the latter is heated. The shadow mask is thermally expanded, resulting in thermal deformation called "doming". The term doming indicates a phenomenon in which the shadow mask is expanded to the side of a fluorescent material.
In doming, the positions of the apertures on the shadow mask are naturally out of their normal positions. The passing electron beam is out of its normal trajectory, reaching a fluorescent material which is not the target of the beam. As a result, the color of a color picture is not given off correctly. This result comes about especially in the case of a large sized CRT wherein the above-described doming results in greater deformation, thereby, degrading the resultant picture quality.
The degradation in images as described above should be prevented. The most general approach for solving this problem is thickening the shadow mask plate. The strength of the shadow mask is also increased by this process. The deformation of the shadow mask is thus less likely to happen, reducing the possibility that the electron beam passes out of its normal trajectory.
The thickening of the shadow mask plate, however, gives rise to another problem. The problem is associated with the etching process for forming apertures on the shadow mask plate. In the etching process, side etching is inevitable. A hole is expanded in the direction vertical to the direction of the thickness of the shadow mask plate. As the thickness of the shadow mask plate increases, a longer time period will be required for forming the apertures by etching. This allows the expansion of the hole in a transverse direction due to the side etching to be even larger. If the spacing between apertures is small increasing mask plate thickness increases the probability that adjacent apertures will be connected to each other due to the side etching. With large-sized color cathode ray tubes being more prevalent in the market, there has arisen a demand for color CRTs with higher resolution. To meet the demand, it is necessary to reduce the spacing of apertures in the shadow mask. If a thick shadow mask plate is used, the recent demand as stated above cannot be met.
A manufacturing method of a shadow mask avoiding the above-described problem and satisfying the recent demand is disclosed, for example, in Japanese Patent Laying-Open No. 49-79170, No. 49-131676, World Patent Laying-Open (National Publication of a Translated Version of International Application) No. WO89/07329, and Japanese Patent Laying-Open No. 1-302639. Referring to FIG. 3, by this method, two shadow mask plates 4 and 5 each having a plurality of apertures 6 and 7 previously formed by means of photoetching are prepared. The shadow mask plates 4 and 5 are piled or staked upon each other so that the apertures 6 and 7 are registered to each other, and shadow mask plates 4 and 5 are joined to each other to produce a shadow mask.
In the above-described methods, a plurality of shadow mask plates should be joined entirely with each other. This is because sufficient strength is particularly needed for a large-sized shadow mask. Otherwise the central portion of the shadow mask will be dented by a shock given to the shadow mask in the manufacturing process of a color cathode ray tube. Further, a large stress is given to the shadow mask in the process of press-molding and, therefore, the apertures on one plate are liable to be displaced from the apertures on the other plate.
A method of joining a plurality of shadow mask plates with each other is disclosed in Japanese Patent Laying-Open Nos. 2-46628, 2-46629, etc. Proposed therein is spot-welding the entire surfaces of the shadow mask plates piled upon each other at several cm intervals, using a laser beam or an electron beam. However, such a method, is accompanied by problems that have yet to be solved for example, assume that shadow mask plates each having a diagonal distance of for example 20 inches are spot-welded at 3 cm intervals. In this case, the shadow mask plates are welded and joined to each other at more than 150 points. The welding for each point itself is completed in a short period of time. Registration for the welding, however, should be carried out precisely. The time required for such registration is supposed to be about 15 seconds for each position. It therefore takes more than 30 minutes to produce one shadow mask by welding two shadow mask plates. Thus, it is seen that the manufacturing process for producing the shadow mask becomes very inefficient so that it is difficult to justify practicing such a process for industrial purposes.